Early detection of smoldering powders in powder drying systems comprising a co gas detection system

ABSTRACT

The invention concerns a powder drying system ( 1 ) comprising a carbon monoxide (CO) gas detection system adapted for detection of CO gas from smoldering powders in a powder drying system component, such as a spray dryer chamber ( 200 ), a fluid bed ( 500 ) or a bag filter ( 400 ), which CO gas detection system comprises at least one inlet CO gas detector ( 3 ) arranged on at least one gas inlet of a powder drying system component such as to provide at least one inlet CO gas content measurement, at least one outlet CO gas detector ( 3 ) arranged on at least one gas outlet of a powder drying system component such as to provide at least one outlet CO gas content measurement, and an analyzing unit ( 5 ) adapted for receiving the at least one inlet CO gas content measurement, receiving the at least one outlet CO gas content measurement and comparing the sum of the at least one inlet CO gas content measurement and the sum of the at least one outlet CO gas content measurement while compensating for dilution, mixing, and time delay of the outlet CO gas content measurement. At least the at least one outlet CO gas detector ( 3 ) comprises an IR laser transmitter and is adapted for detecting over a measurement volume ( 6 ) and is arranged on the at least one gas outlet in such a way that said measurement volume ( 6 ) extends directly inside a gas flow ( 20 ) in said at least one gas outlet.

FIELD OF INVENTION

The present invention relates to a powder drying system with a systemadapted for early detection of smoldering powders in a powder dryingsystem component, such as a spray dryer chamber, a fluid bed or a bagfilter, by detecting the carbon monoxide (CO) gas emitted duringsmoldering.

Note that the term “measurement volume” as used herein is intended torefer to the volume, area, line, and/or point from which the CO gasdetection system receives measurement information is arranged, or inother words is covered by a measurement performed by the CO gasdetection system.

Note that the term “measurement range” as used herein is intended torefer to the position in which the measurement volume is to be arranged,and may thus encompass not only the actual measurement volume but also azone or buffer zone immediately adjacent to the measurement volume.

Note that the term “gas” as used herein is to be interpreted asencompassing any gas that is suitable as process gas in such a spraydrying system. Some non-limiting examples of gasses are process gasses,such as atmospheric air and suitable oxygen- or nitrogen-enhanced gases.

Note that the term “powder drying system” is intended to encompass suchsystems in which a powdery or particulate material is formed and/orprocessed. The material may either be provided as a feed of powdery orparticulate material, or as a liquid feed to be dried. The powder dryingsystem is also intended to cover cooling of the particulate material. Inaddition, or alternatively, to the spray dryer described, such a systemcould include one or more fluid beds, cyclones, bag filters, pneumatictransport systems, feed pretreatment etc. The powder drying system thusincorporates a unit for forming or processing powder in any suitablemanner. Non-limiting examples encompass a spray dryer, fluid bed, flashdryer, ring dryer, spray dryer with integrated or external fluid bed,cyclone, etc. In general, a powder drying system has an inlet for aliquid, paste or solid feed e.g. at the top of a vertical dryer, where aliquid feed enters the dryer and meets a stream of dry, hot air so thatdroplets of feed will fall down inside the dryer and liquid will beremoved from the droplets resulting in formation of substantially dryparticles. The liquid feed may be a slurry or suspension of particles ora solution capable of forming particles upon removal of the solvent.

Note that the term “powder drying system component” is intended toencompass any component of a powder drying system in which a process gasis present at least during operation, such as but not limited to dryingchambers, fluid beds, bag filters, cyclones, ducts, such as cleaningarrangements connecting ducts.

Note that the term IR laser transmitter is intended to encompass anylaser light source, such as a laser setup or diode that can emit IRradiation in the IR wavelength band, defined by electromagneticradiation with wavelengths in the range of around 700 nm to around 1 mm.

Note that the term IR receiver is intended to encompass any detectorthat can detect IR radiation.

BACKGROUND OF THE INVENTION

Smoldering powders, so-called nests, i.e. particulate burning embers andagglomerates, are common ignition sources in dust explosions and fires,and pose a serious threat in powder drying systems, such as spraydryers, that produce powders within food, pharmaceuticals, dairy,chemical, agro-chemical, energy, biotechnology, healthcare and manymore, producing e.g. milk powder, coffee whitener, infant formula,coffee powder, pharmaceutical, chemical, etc. During a spray dryingprocess deposits may occur when generally not-high-temperature-dropletsaccumulate in smaller or larger deposits on walls or floors of driers.The product temperature of such deposits may rise due to intrinsicchemical reactions, which may cause the powder to smolder, or even burnwhen in oxygen filled atmospheres. During smoldering, CO gas productionstarts, and an oxidation and/or pyrolysis reaction occurs. Lumps ofdeposited smoldering product may typically develop inside thespray-chamber and/or air disperser/ and/or atomizer, cyclone or in aninterior or exterior fluid bed, or in exterior bag filters.

If powder deposits are not removed from the drying system and reach acertain size, they may form lumps and fall down inside the system, eventravel through the process and break-up so a so-called glowing mass,initiated by heat as an exothermal oxidation reaction. The glowing massmay become exposed and ignite a powder filled atmosphere at some pointin time. Another scenario is that larger lumps of powder are subjectedto heat, e.g. at the bottom of a fluid-bed and smoldering reactions areactivated. This can take place also in low oxygen atmospheres or thelike. Subsequently, the smoldering lump may travel through the processand finally result in plant fires or dust explosions. Primary fireprevention is typically based on temperature surveillance and regularcleaning practices. Often, such primary prevention is not fast enoughdue to process reaction delay and therefore insufficient; and additionalmeasures are implemented in the form of smoldering detection systems forearly warning and protection of the drying system.

One prior art solution is produced and marketed by FIKE under the nameWarnEx. This system consists of multiple so-called sampling anddetection units (SDUs) placed on each inlet and outlet of a powderdrying system and a control unit processing the signals received fromthe SDUs. The WarnEx system thus employs extraction of samples in thesampling part of an SDU and transport of the detection result to thedetection part of the SDU through suitable electric cabling.

DE 202014101777 U1 describes a spray dryer with a humidity detectorhaving a measuring device, which humidity detector may also be used fordetection of CO gas. The specific type of measuring device and measuringmethod employed in detection of CO gas by means of the humidity detectoris, however, not mentioned.

Furthermore, DE 202014101777 U1 and other known CO gas detection systemsfor spray dryers employ extraction of gas samples from the flow at leastat an inlet and at an outlet thereof, and transport these gas samplesvia e.g. Teflon® tubes to a common IR laser or a common NDIR detectorsystem (not laser) provided at a distance from the chamber. Such CO gasdetection systems are produced and marketed by e.g. Hobre Instrumentsand ATEX CO.

However, the known CO gas detection systems have several disadvantages,including:

-   -   reduced result reliability when measuring on a small volume of        gas in a gas sample,    -   the sample may not be representative for the actual CO gas        concentration in the flow out or in as it depends on the airflow        and dilution at the selected extraction position,    -   they require accurate timing between the physical gas sample        from the in/outlet in the system,    -   need for frequent calibration to ensure the required sensitivity        at all times, in particular when the flow rates or volume size        of drier changes,    -   high acquisition, installation, operation, maintenance and        verification/calibration costs, and    -   is difficult to adapt to changes in flow or volume size of        dryer, and    -   requires external control signals from another source.

Furthermore, the prior art systems, such as those of ATEX CO and Hobé,as mentioned employ tubing to transport the gas samples from thesampling site to the measurement site, i.e. the common externaldetector. Such tubing requires maintenance, may leak, is ergonomicallydifficult to handle when installing and requires a large detector systemcabinet and high power consumption. Additionally, their system cabinet(of the floor standing type) requires certain conditions for theinstallation area, like the temperature shall be below 25° C., it has tobe installed in a clean confined environment and access to the cabinetfrom several sides, and a water drain is necessary for drainage ofcondensate coming from the system. Also, humidity in the samples must beremoved before measurement, and it is needed for the detector systemcalculation to be compensated for e.g. different geometries or flowsinside the tubes, so that repairs on tubes and connectors cannot beperformed by plant staff but must be undertaken by professionals, e.g.detector providers.

The response time of a CO gas detection system should be low andpreferably real time and the sensitivity high in order to provide aneffective early warning system.

However, the transport time of the air samples, added to the detectorpurge time, analysis time, calculation time and the intrinsic holdingtime in the common measurement chamber adds up to longer response timesfor these prior art systems of around 15-60 seconds. It is desirable todecrease this response time, which may prove too long to constituteearly warning. As for sensitivity, some of the prior art systems areincapable of detecting CO gas concentrations of generally below about 1ppm and for the best prior art systems below about 0.4 ppm. To providefor an effective early warning system it is, however, desired to providea sensitivity enabling detection of CO gas concentrations generallybeing below 0.4 ppm and preferably below 0.1 ppm.

During initial commissioning of prior art CO gas detection systems forspray dryer systems, tests are needed where hazardous CO gas areinjected into the entire dryer system for testing the gas retention timefor necessary calibration procedure of the CO gas detection.Importantly, the purchase and handling of CO gas normally requires localregulatory approval. With some of the prior art systems as stipulated bythe spray dryer systems risk assessment or to ensure optimal systemperformance require an enhanced test carried out frequently on thesystem before a spray dryer system is released for operation. One of thesteps in this test is use of a special certified test gas consisting ofN₂+CO (CO=8.0 ppm) to confirm that the CO gas detection is measuringcorrectly. The test gas is expensive and not commonly available on themarket. Another requirement is leak tests of sample tubes for prior artCO gas detection systems. This test must frequently be carried out toensure that the CO gas detection system actually is measuring theprocess gas coming from inside the dryer system.

A further problem resides in the presence of invading spurious CO gasfrom external sources, in particular coming from climate fluctuations,varformed in ambient air or exhaust gases from cars, field burning ofcrops, human activity, etc., which causes the ambient CO gas, whichenters into the system air, to vary greatly according to location andpollution in the plant, area, country and weather. Many decades largerCO gas content may thus be present in the inlet air and mask oroverpower the CO gas generated from any smoldering powder inside theprocess. This problem is attempted solved for the prior art systems byusing a reference ambient air sample and/or by calculating thedifference between CO gas content in inlet(s)- and outlet(s) samplesusing a fixed reference CO gas value or a differential calculation, i.e.a subtraction of the summed CO gas contents from all inlets and thesummed CO gas contents from all outlets.

SUMMARY OF THE INVENTION

With this background, it is therefore an object of the invention toprovide a powder drying system adapted for early detection of smolderingnests by means of a carbon monoxide (CO) gas detection system, withwhich powder drying system the above mentioned problems anddisadvantages are mitigated.

In a first aspect of the invention, these and further objects areobtained by a CO gas detection system adapted for detection of CO gasfrom smoldering powders in a powder drying system component, such as aspray dryer chamber, a fluid bed or a bag filter, which CO gas detectionsystem comprises at least one inlet CO gas detector arranged on at leastone gas inlet of a powder drying system component such as to provide atleast one inlet CO gas content measurement, at least one outlet CO gasdetector arranged on at least one gas outlet of a powder drying systemcomponent such as to provide at least one outlet CO gas contentmeasurement, and an analyzing unit adapted for receiving the at leastone inlet CO gas content measurement from the at least one inlet CO gasdetector, receiving the at least one outlet CO gas content measurementfrom the at least one outlet CO gas detector and comparing the sum ofthe at least one inlet CO gas content measurement and the sum of the atleast one outlet CO gas content measurement while compensating fordilution, mixing, and time delay of the outlet CO gas contentmeasurement such as to provide a differential measurement, ΔC_(outlet),indicative of the CO gas content from smoldering powders in a powderdrying system component, where at least the at least one outlet CO gasdetector comprises an IR laser transmitter and is adapted for detectingover a measurement volume and is arranged on the at least one gas outletin such a way that said measurement volume extends directly inside a gasflow in said at least one gas outlet.

Other components of a powder drying system to be surveyed could becyclones, ducts and the like. It is even possible that the exterior of apowder drying system, i.e. the room in which the powder drying system ora component thereof is located, could be surveyed.

The CO gas detection system according to the invention thus employs asmeasurement principle absorption of an IR laser beam at a specificwavelength emitted and detected by the IR detector. For instance, theabsorption spectrum of CO exhibits major peaks at around 2.3 μm andaround 4.7 μm. The specific wavelength of the IR laser beam emitted anddetected by the IR detector is therefore typically around 2.2 to 2.4 μm.The adsorption is proportional to the number of CO molecules in the gas,and therefore the concentration of CO gas in the gas can be calculatedfrom the adsorption, pressure and temperature of the gas. Furthermore,as the absorption spectrum of methane (CH₄) exhibits a major peak ataround 2.2 μm, the CO gas detection system may also be employed fordetecting CH₄ gas.

By providing at least the at least one outlet CO gas detector as anIR-laser detector suitable for detecting over a measurement volume andarranging it on the gas outlet in such a way that its measurement volumeextends directly inside a gas flow in the gas outlet, direct measurementof the CO gas concentration at the measurement site is obtained in realtime. Thus, only the result of the measurement needs to be transferredto the analyzing unit, which may be done by simply transferringelectronic or optical signals over a wired or, even better, wirelesselectronic or optical communication line. In other words, with thesystem according to the invention, no sample collection, samplepreparation and sample transport is necessary. Consequently, there is noneed for costly, time consuming, and complex sampling equipment andtubing system for transport of the gas samples to the analyzing unit.This in turn provides for a system comprising very few components andthus being cost efficient, and simple to install and retrofit. Also, theprocess flow in the spray dryer system parts is completely undisturbedby the measurement process, and the quality of the spray dried productis thus not influenced.

A further advantage lies in that the direct measurement of the CO gasconcentration at the measurement site and digital, such as optical orelectronic transfer of the result to the analyzing unit makes itpossible to reduce the response time of the system, i.e. the time delaybetween individual measurements very considerably, and in practice to aslow as about 1 second and can even be lower. This applies to all sizesof spray dryer systems, and is much lower than for the prior artsystems.

Furthermore, with a system according to the invention only one initialcalibration procedure is needed for commissioning of the CO gas detectorand the calibration may be performed by injection of methane gas intothe entire system, which is easier to acquire than the certified N2+COgas. Therefore, the need for the specialists aside of the detectorprovider to perform calibration and handle the hazardous CO gas bottlesfor commissioning is eliminated. Thus, the system according to theinvention is also both simple and cost efficient in maintenance, saferand easier to commission. The verification of the invention's measuringaccuracy is only required once a year. If verification shows that theinvention need adjustment/calibration, the N₂+CO (CO=8.0 ppm) gasmixture or methane is used for that.

By providing an analyzing unit adapted for comparing the inlet CO gascontent measurement and the outlet CO gas content measurement tocompensate for dilution, mixing, and time delay of the outlet CO gascontent measurement such as to provide a differential measurement,ΔC_(outlet), indicative of the CO gas emanating from smoldering powdersin a powder drying system component, it is made possible to take intoaccount both spurious CO gas from external sources and dilution, mixingand time delay in the powder drying system into consideration in theanalysis and thus the true CO gas content measurement. Thereby a CO gasdetection system is provided with which the reliability and sensitivitywhen measuring on a small volume of gas is increased considerably, andthe resulting CO gas measurement is very precise and highly reliable.

Indeed, experiments performed on a powder drying system with a CO gasdetector system according to the invention has shown that thesensitivity of the system is sufficiently high to detect CO gasconcentrations from smoldering nests of 1 ppm (parts per million) orbelow. For instance, it has been shown that for a large powder dryingsystem CO gas concentrations of as low as 0.3 to 0.4 ppm may bedetected, while as high a sensitivity as corresponding to detectabilityof CO gas concentrations of as low as 0.04 ppm has been proved. Thesensitivity and precision of the measurements is crucial in order tohave an early warning system. This is considerably lower than the priorart systems.

The lowered response time and the increased sensitivity in turn providesfor an effective early warning enabling action to be taken at a veryearly point of time in case smoldering begins to take place.

In an embodiment the at least one inlet CO gas detector also comprisesan IR laser transmitter and is adapted for detecting over a measurementvolume and is provided on the at least one gas inlet in such a way thatits measurement volume extends directly inside a gas flow in said gasinlet.

Thereby, a CO gas detection system is provided in which advantagessimilar to those described above are also achieved when obtaining theinlet CO gas measurement, and the sensitivity of the system is improved.

In an embodiment at least the at least one outlet CO gas detector and/orat least the at least one inlet CO gas detector comprises an IR lasertransmitter and an IR receiver, and optionally a reflector the IR lasertransmitter and the IR receiver being arranged on positions being one ofmutually off-set in a radial and/or longitudinal direction, mutuallyopposite and diametrically opposite such that its measurement volumeextends between said positions.

In a further, optional, embodiment at least the at least one inlet COgas detector and/or outlet CO gas detector comprises an IR lasertransmitter and an IR receiver and at least one reflector, the IR lasertransmitter and the IR receiver being arranged in the same position andthe at least one reflector being arranged in a position being one ofoffset in a radial and/or longitudinal direction, mutually opposite anddiametrically opposite with respect to the position of the IR lasertransmitter and/or the IR receiver such that the measurement volume ofthe at least one inlet and/or outlet CO gas detector system extendsbetween said positions.

By any of the two above mentioned embodiments a CO gas detection systembeing extremely simple in construction and very simple to mount,including retrofit on an existing powder drying system, is provided for.Such a CO gas detection system is also cost effective in bothprocurement and in maintenance. By using reflectors, the measurementvolume can be increased, and thus improve the sensitivity of the system.

In an embodiment the measurement volume of the outlet CO gas detectorand/or the measurement volume of the inlet CO gas detector extends overa length of at least 1 meter.

Thereby, a measurement volume is provided which is sufficiently large toobtain a CO gas content measurement being both highly representative ofthe CO gas concentration in the inlet or outlet and also enabling therequired sensitivity. It is noted that the optimum length depends on thetype and effect of the IR laser transmitter and/or IR receiver beingused and may thus vary according thereto.

In an embodiment the measurement volume of the outlet CO gas detectorand the measurement volume of the inlet CO gas detector may extend alongany one of a radial direction of the gas inlet or gas outlet, along alongitudinal direction of the gas inlet or gas outlet perpendicular tothe radial direction and a direction being inclined with respect to thelongitudinal direction and/or the radial direction of the gas inlet orgas outlet.

Thereby, a CO gas detection system which is simple to mount on a powderdrying system is obtained while simultaneously in a particularly simplemanner ensuring that a measurement volume is provided which issufficiently large to obtain a CO gas content measurement being bothhighly representative of the CO gas concentration in the inlet or outletand also enabling the required sensitivity.

In an embodiment the at least one outlet CO gas detector and/or the atleast one inlet CO gas detector further comprises at least one purgingdevice arranged and adapted for purging the CO gas detector prior toproviding the CO gas content measurement.

By providing a purging device, residues of powder originating from thespray drying process which could accumulate on the outlet CO gasdetector and/or the inlet CO gas detector, particularly on IR receiverand the optical components placed in the duct, e.g. lenses, of the COgas detector system, over time may be removed or such powderaccumulation may even be avoided altogether. This in turn provides forthe removal of noise and disturbances in the measurements originatingfrom such powder accumulation, thus improving measurement quality evenfurther.

Advantageously, the gas used by the purging device for purging the COgas detector comprises no CO gas. Thus, at least a small part of themeasurement range immediately in front of the CO gas detector in thesystem component in which the CO gas detector's measurement volume is tobe positioned is kept free from retained CO, which could have aninfluence on the accuracy of measurement. Thereby, a well-defined startof the measurement volume containing the concentration of CO gas to bemeasured will be provided. If, on the other hand, CO gas is present inthe gas used by the purging device for purging the CO gas detector, thisCO gas will also be detected by the CO gas detector and thus add anunwanted contribution to the measurement.

In an embodiment at least the inlet CO gas detector and/or the outlet COgas detector is cooled, preferably air cooled. Thereby, it becomespossible to avoid drift of the measured spectrum of the IR laser beamdue to heating. It is noted that in embodiments where the outlet and/orinlet CO gas detector comprises a purging device, the purging device maysimultaneously serve to air cool the CO gas detector.

In an embodiment the powder drying system further comprises a pluralityof outlet CO gas detectors arranged on the same gas outlet and/or onoutlets of different powder drying system components such as to providea plurality of outlet CO gas content measurements, and/or a plurality ofinlet CO gas detectors arranged on the same gas inlet and/or on inletsof different powder drying system components such as to provide aplurality of inlet CO gas content measurements.

Thereby, it becomes possible to detect CO gas concentrations originatingfrom smoldering of powders at several sites simultaneously. This in turnprovides for a further improved early warning while still keeping thesystem simple and cost effective in both structure, mounting andmaintenance.

In an embodiment the analyzing unit is adapted for comparing the inletCO gas content measurement and the outlet CO gas content measurement toobtain a differential measurement, ΔC_(outlet)(x), indicative of CO gasfrom smoldering powders in a powder drying system component at a giventime x by means of the relation:

ΔC _(outlet)(x)=C _(outlet,measured)(x)−C _(outlet)(x),

where:

${{C_{outlet}(x)} = \frac{{\left( \frac{{c_{inlet}\left( {x - t_{p}} \right)} \star {FLOW}_{inlet}}{V_{spraydyer}} \right) \star t_{s}} + {C_{outlet}\left( {x - t_{s}} \right)}}{1 + \left( \frac{{FLOW}_{outlet} \star t_{s}}{V_{spraydryer}} \right)}},$

and where:

C_(outlet,measured)(X) is the concentration of CO gas in ppm in the gasoutlet according to an outlet CO gas content measurement as measured bymeans of the outlet CO gas detector at the time x,

C_(outlet)(X) is the calculated concentration of CO gas in ppm in thegas outlet at the time x,

C_(outlet)(x-t_(s)) is the calculated concentration of CO gas in ppm inthe gas outlet at the time x minus the sampling time t_(s), i.e. thelatest previously calculated value of the concentration of CO gas in ppmin the gas outlet,

C_(inlet)(x-t_(p)) is the concentration of CO gas in ppm in the gasinlet according to an inlet CO gas content measurement as measured bymeans of the inlet CO gas detector a number of seconds t_(p) before thetime x,

V_(spraydryer) is the volume of the spray dryer in m³,

FLOW_(inlet) is the air flow in m³/s in the main air inlet,

FLOW_(outlet) is the total gas flow out of the spray dryer, calculatedas the sum of the gas flow in each of the main gas outlet, the gasoutlet of the static fluid bed and the gas outlet of theVIBRO-FLUIDIZER™ in m³/s, and

t_(s) is the sampling time.

Thereby it becomes possible not only to take into account spurious COgas from external sources and dilution, mixing and time delay in thepowder drying system, but to do so continuously from production run toproduction run or measurement to measurement by dynamic modeling of thepowder drying system, rather than—as in the prior art systems—using afixed compensation value. Also, the above described model has provenparticularly precise for all sizes of powder drying systems or plantsand it is an advantage that flow and volume changes easily can beentered into the model, e.g. directly acquired from the PLC-system.

In an embodiment the inlet CO gas detector(s) and/or outlet CO gasdetectors including, where provided, the purging device(s) may bemounted on inlet and/or outlet flanges welded to the inlet and/or outletducts. Thereby a simple, quick and durable mounting of the CO gasdetectors is provided for.

Thereby the measurement range may be cleaned for components which mayotherwise influence the CO gas content measurement negatively before theCO gas content measurement is carried out, e.g. by changing the lengthof the optical beam path through the inlet or outlet. This in turnprovides for an improved measurement sensitivity.

According to the invention, the above and further objects arefurthermore achieved by means of a method for detecting CO gas fromsmoldering powders in a powder drying system component, such as a spraydryer chamber, a fluid bed or a bag filter, of a powder drying systemcomprising a carbon monoxide (CO) gas detection system, the methodcomprising the steps of providing an inlet CO gas content measurement bymeans of an inlet CO gas detector arranged on an gas inlet of a powderdrying system component, providing an outlet CO gas content measurementby means of at least one outlet CO gas detector arranged on an gasoutlet of a powder drying system component, receiving the inlet CO gascontent measurement from the inlet CO gas detector, receiving the outletCO gas content measurement from the at least one outlet CO gas detector,comparing the inlet CO gas content measurement and the outlet CO gascontent measurement to compensate for dilution, mixing, and time delayof the outlet CO gas content measurement such as to provide adifferential measurement, ΔC_(outlet), indicative of CO gas fromsmoldering powders in a powder drying system component, where the atleast one outlet CO gas detector comprises an IR laser transmitter andis adapted for detecting over a measurement volume and is, prior toproviding the outlet CO gas content measurement, arranged on the gasoutlet in such a way that its measurement volume extends directly insidea gas flow in the gas outlet.

Further features of a method according to the second aspect of theinvention will be apparent from the below detailed description and areset forth in the dependent method claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail below by means ofnon-limiting examples of presently preferred embodiments and withreference to the schematic drawings, in which:

FIG. 1 shows a schematic view of the main components of a spray dryingsystem in an embodiment of the first aspect of the invention, differentexemplary positions of a CO gas detection system according to theinvention being indicated;

FIG. 2 shows a schematic view of a CO gas detection system in anembodiment of the invention, where the CO gas detection system isarranged on an outlet of the spray drying system;

FIG. 3 shows a cross sectional view of a CO gas detection systemaccording to FIG. 2; and

FIG. 4 illustrates schematically the steps of a method according to anembodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 shows a schematic view of the main components of a powder dryingsystem comprising a powder processing unit which by way of example inthe embodiment shown is in the form of a spray drying system 1. In amanner known per se, the spray drying system 1 comprises a spray dryerwith a drying chamber 200 and a process gas supply device 300, typicallyincluding an gas disperser. A gas inlet 201 is provided for intake ofgas to the process gas supply device 300 and further to the dryingchamber 200. At the lower end of the drying chamber 200, an outlet 203for dried material is provided, and furthermore, for some types ofpowder drying systems, a further gas inlet 201′ may be present. Thedrying chamber 200 also incorporates atomizing means, such as nozzlesand/or a rotary atomizer wheel.

In the shown spray drying system 1, a static fluid bed 700 with airinlet 701 is provided and an after-treatment unit in the form ofvibrating or static fluid bed 500 is provided. At one end, the vibratingor static fluid bed 500 comprises an inlet 503 at which it receivesdried material from the outlet 203 of the drying chamber 200 for furthertreatment of the material, which is then to be collected at an outlet504 at the other end of the vibrating or static fluid bed 500. The fluidbed 500 further comprises gas inlets 501 and 501′ as well as a gasoutlet 502. Further upstream or downstream equipment may be present aswell.

Furthermore, the powder drying system comprises in addition to the spraydrying system 1 a filter unit 400, to which spent process gas withparticles entrained in the process gas is conducted. The filter unit 400thus comprises a gas inlet 401 for spent process gas from one or more ofthe upstream operational units, a plurality of bag filters and a cleanair outlet 601. The filter unit 400 may form part of a series of powderrecovery units including further filter units and cyclones or bagfilters, or any combination thereof. Furthermore, a cleaning arrangement600 is shown in FIG. 1.

A number of conveying lines connect the operational units with eachother in a manner known per se and will not be described in detail.

The general configuration of the CO gas detection system of a spraydrying system according to the invention will now be described in moredetail with particular reference to FIGS. 2-3.

According to the invention the spray drying system 1 further comprises acarbon monoxide (CO) gas detection system adapted for detection of COgas from smoldering powders in a spray drying system component, such asfor instance the spray dryer chamber 200, the fluid bed 500 or the bagfilter unit 400.

Generally, the CO gas detection system comprises at least one CO gasdetector arranged on a gas outlet, herein denoted outlet CO gasdetector, and at least one CO gas detector arranged on a gas inlet,herein denoted inlet CO gas detector. In FIG. 1 various exemplary andnon-limiting possible positions of outlet CO gas detectors and inlet COgas detectors are indicated. Since the at least one outlet CO gasdetector and the at least one inlet CO gas detector is of identical orsimilar structure, both are indicated on FIG. 1 by the reference numeral3, and FIGS. 2-3 illustrate an embodiment of a CO gas detector which byway of example is arranged on a gas outlet 202, but which may just aswell have been arranged on a gas inlet.

Generally, the CO gas detection system comprises an outlet CO gasdetector 3 arranged on an gas outlet, such as on an outer surfacethereof, of a powder drying system component such as to provide anoutlet CO gas content measurement. Referring to FIGS. 2 and 3, the COgas detection system more particularly and by way of a non-limitingexample comprises an outlet CO gas detector 3 arranged on the gas outlet202 of the drying chamber 200 of the spray drying system 1. Moreparticularly, the outlet CO gas detector 3 is arranged on an outersurface 2021 of the gas outlet 202 of the drying chamber 200. Generally,an outlet CO gas detector 3 may alternatively or additionally bearranged on a second or further gas outlet of the same or a second spraydrying system component, such as those described above and/or such asindicated in FIG. 1.

The CO gas detection system may further comprise at least one inlet COgas detector arranged on a gas inlet, such as on an outer surfacethereof, of a powder drying system component such as to provide an inletCO gas content measurement. The CO gas detection system moreparticularly and by way of a non-limiting example comprises an inlet COgas detector arranged on the gas inlet 201 of the drying chamber 200 ofthe spray drying system 1. More particularly, the inlet CO gas detectoris arranged on an outer surface of the gas inlet 201 of the dryingchamber 200. Generally, one or more inlet CO gas detectors may,alternatively or additionally, be arranged on one or more gas inlets ofother spray drying system components, such as those described aboveand/or such as indicated in FIG. 1. In a preferred embodiment, the inletCO gas detector measuring IR radiation.

For instance, the outlet CO gas detector 3 and/or the inlet CO gasdetector, respectively, may in some embodiments be mounted on flangeswelded to the gas outlet and/or gas inlet, respectively.

The CO gas detection system further comprises an analyzing unit 5. Theanalyzing unit 5 is adapted for receiving the inlet CO gas contentmeasurement from the inlet CO gas detector and for receiving the outletCO gas content measurement from the at least one outlet CO gas detector3. The analyzing unit 5 may thus comprise a receiver. The analyzing unit5 is furthermore adapted for comparing the inlet CO gas contentmeasurement and the outlet CO gas content measurement to compensate fordilution, mixing, and time delay of the outlet CO gas contentmeasurement such as to provide a differential measurement, ΔC_(outlet),indicative of CO gas from smoldering powders in a spray drying systemcomponent, i.e. in the embodiment illustrated in the spray dryingchamber 200. The analyzing unit 5 is thus electrically and/or opticallyconnected to the inlet CO gas detector and the outlet CO gas detector 3by means of a respective wired or wireless connection 13 (FIG. 2).

The analyzing unit 5 is furthermore adapted for, in case the result ofthe comparison shows an indication of CO gas from smoldering powders ina spray drying system component, taking an appropriate action. Such anappropriate action may comprise setting off an alarm, e.g. in the formof an acoustic or a visual signal, and/or automatically shutting downthe spray drying system and/or activating fire-preventing means.

The analyzing unit 5 may thus, and as is shown on FIG. 2, form anintegrated or stand-alone part of a safety system, which also comprisesan alarm unit, e.g. in the form of an acoustic or a visual signalingunit. The alarm unit may alternatively be built into the analyzing unit5. In any event, the analyzing unit is adapted for triggering the alarmunit and the alarm unit is adapted for emitting an alarm when thedifferential measurement, ΔC_(outlet), indicative of CO gas fromsmoldering powders in a spray drying system component is above apredetermined threshold value, typically being close to 0. Furthermore,the alarm unit may also be triggered if the analysis shows an erroneousfunction of the CO gas detection system, particularly to emit a signaldedicated to the purpose, which is a feature not being possible in theprior art system. The analyzing unit 5 is thus electrically and/oroptically connected to the alarm unit by means of a wired or wirelessconnection.

Referring to FIGS. 2 and 3, the outlet CO gas detector 3 is an IRdetector suitable for detecting over a measurement volume 6. The outletCO gas detector 3 is arranged on the outlet 202 in such a way that itsmeasurement volume 6 extends directly inside a (process) gas flow 20 inthe outlet 202. More particularly, the outlet CO gas detector 3comprises an IR laser transmitter 7 and an IR receiver 8. The IR lasertransmitter 7 and the IR receiver 8 are arranged on opposite points,here but not mandatorily always on diametrically opposite points, on theoutlet 202 such that its measurement volume 6, i.e. the volume throughwhich the IR laser beam 15 of the IR detector extends when in operationand measuring, extends over a length D between the opposite points ofthe outlet 202 through the inside 2022 of the outlet 202. The IR lasertransmitter 7 and the IR receiver 8 may furthermore be electricallyand/or optically connected by means of a wired or wireless connection12.

In alternative embodiments the outlet CO gas detector 3 may alsocomprise one or more IR reflectors, which reflectors may be arrangedtogether with the IR laser transmitter, e.g. in the same housing, orapart from the emitter, e.g. in a different housing than the IR lasertransmitter, such as in the housing of the IR receiver, or to redirectthe laser light over one or more segments along one or more of thelongitudinal direction, the radial direction and the cross sectionaldirection of the gas outlet. The same applies to the inlet CO gasdetector.

The outlet CO gas detector 3 may further comprise at least one purgingdevice 9 arranged and adapted for purging the outlet CO gas detector 3prior to providing the outlet CO measurement. The purging device 9 isconnected to the outlet CO gas detector 3, particularly to the IR lasertransmitter 7 and to the outlet IR receiver 8, by means of an air tube4.

The inlet CO gas detector may also be substantially identical to theoutlet CO gas detector 3. The inlet CO gas detector may preferablycomprise an IR laser transmitter and is adapted for detecting over ameasurement volume.

The inlet CO gas detector is then provided on the inlet 201 in such away that its measurement volume extends directly inside a (process) gasflow in the inlet 201. More particularly, the inlet CO gas detectorsystem comprises an IR laser transmitter and an IR receiver. The IRlaser transmitter and the IR receiver are arranged on opposite points,possibly but not mandatorily diametrically opposite points, on the inlet201 such that its measurement volume, i.e. the area in which the IRlaser beam of the IR detector extends when in operation and measuring,extends over a length between the opposite points of the inlet 201through the inside of the inlet 201. The IR laser transmitter and the IRreceiver may furthermore be electrically and/or optically connected bymeans of a wired or wireless connection.

The inlet CO gas detector may further comprise at least one purgingdevice arranged and adapted for purging the inlet CO gas detector priorto providing the inlet CO gas content measurement. The purging device isconnected to the inlet CO gas detector, particularly to the IR lasertransmitter and to the inlet IR receiver, by means of air tubes.

The measurement volume 6 of the outlet CO gas detector 3 and themeasurement volume of the inlet CO gas detector may extend along any oneof a radial direction R (FIG. 2) of the gas inlet or gas outlet as shownin FIG. 3, along a longitudinal direction L (FIG. 2) of the gas inlet orgas outlet perpendicular to the radial direction R and a direction beinginclined with respect to both the longitudinal direction L and theradial direction R of the gas inlet or gas outlet. The measurementvolume 6 of the outlet CO gas detector 3 and the measurement volume ofthe inlet CO gas detector may extend over a length of more than 1 meter,such as to provide for a measurement volume being sufficiently large toenable the desired sensitivity and precision of the measurements.

The measurement principle employed by the invention is thus adsorptionof an IR laser beam at specific wavelengths emitted from the IR lasertransmitter 7. The adsorption will be proportional to the number ofmolecules of CO in the process gas. Thus, from the adsorption as well asfrom the temperature and pressure of the process gas, the concentrationof CO gas may be calculated based on analysis of the resultingadsorption spectrum, for instance in the manner described further below.

In a manner known per se to the skilled person within IR based CO gascontent measurements, the outlet CO gas detector 3 may further comprisea first transmitter 10 for measuring process gas temperature forproviding a process temperature compensation to the CO gas contentmeasurement to compensate for the effects on the measurements caused byelevated temperatures caused by the IR laser beam, or in particular fromelevated process temperatures compared to room temperature, which mayotherwise shift the adsorption spectrum obtained. The outlet CO gasdetector 3 may further comprise a second transmitter for 11 measuringprocess gas pressure for providing process pressure compensation to thecalculation to compensate for the effects on the measurements caused byelevated process pressures compared to atmospheric pressure.Analogously, the inlet CO gas detector may further comprise a firsttransmitter for providing process temperature compensation and/or asecond transmitter for providing process pressure compensation.

Furthermore, in some embodiments where the CO gas detection systemcomprises a purging device 9, the purging device 9 may also be arrangedand adapted for purging a measurement range 6′ (FIG. 3) inside the gasoutlet before providing the outlet CO gas content measurement and/or bearranged and adapted for purging a part of a measurement range insidethe gas inlet before providing the inlet CO gas content measurement.

Still further, the outlet CO gas detector 3 and/or the inlet CO gasdetector, such as comprising the IR laser transmitter and or IRreceiver, may in some embodiments be cooled, such as air cooled or evenwater cooled. For instance, the purging units may also serve as aircooling units. Alternatively, a separate cooling device may be provided.

Referring now to FIG. 4 exemplary embodiments of a method according tothe invention will be described.

Prior to step 1000 a spray drying system with a CO gas detection systemaccording to the invention is provided by mounting said CO gas detectionsystem on the spray drying system. In step 1000, an inlet CO gas contentmeasurement is provided by said at least one inlet CO gas detector. Instep 1200 an outlet CO gas content measurement is provided by means ofsaid at least one outlet CO gas detector. In step 1300 the inlet CO gascontent measurement from the inlet CO gas detector is received by theanalyzing unit 5. In step 1400 the outlet CO gas content measurementfrom the at least one outlet CO gas detector is received at theanalyzing unit 5. In step 1500 the inlet CO gas content measurement andthe outlet CO gas content measurement is compared by means of theanalyzing unit 5 to compensate for dilution, mixing, and time delay ofthe outlet CO gas content measurement such as to provide a differentialmeasurement, ΔC_(outlet), indicative of CO gas from smoldering powdersin a spray drying system component. Finally, in step 1100, prior to thestep 1200 of providing the outlet CO gas content measurement, the atleast one outlet CO gas detector is provided as an outlet CO gasdetector comprising an IR laser transmitter and being adapted fordetecting over a measurement volume and is arranged on the gas outlet insuch a way that its measurement volume extends directly inside a gasflow 20 in the inside 2022 of the gas outlet 202.

The method may furthermore comprise one or more of the followingoptional steps.

A step 900 of, prior to the step 1000 of providing the inlet CO gascontent measurement, providing the inlet CO gas detector as an inlet COgas detector optionally comprising an IR laser transmitter and beingadapted for detecting over a measurement volume, and arranging the inletCO gas detector on the gas inlet in such a way that its measurementvolume extends directly inside a (process) gas flow in the inside of thegas inlet 201.

In embodiments where at least the outlet CO gas detector comprises an IRlaser transmitter and an IR receiver, a step of arranging the IR lasertransmitter and the IR receiver on opposite points, optionallydiametrically opposite points, on the gas outlet 202, such as on anouter surface 2021 of the outlet 202, such that its measurement volumeextends between the opposite points of the gas outlet 202.

In embodiments where the inlet CO gas detector comprises an IR lasertransmitter and an IR receiver, a step of arranging the IR lasertransmitter and the IR receiver on opposite points, optionallydiametrically opposite points, on the gas inlet 201, such as on an outersurface of the inlet 201, such that its measurement volume extendsbetween the opposite points of the gas inlet 201.

An optional step of purging the outlet CO gas detector prior to the step1200 of providing the outlet CO gas content measurement.

An optional step of purging the inlet CO gas detector prior to the step1300 of providing the inlet CO gas content measurement.

An optional step of cooling, preferably air cooling, the outlet CO gasdetector and/or a step of cooling, preferably air cooling, the inlet COgas detector.

An optional step of providing a plurality of outlet CO gas detectors,arranging the plurality of outlet CO gas detectors on the same gasoutlet and/or on outlets of different spray drying system components,and a step of providing a plurality of outlet CO gas contentmeasurements.

An optional step of providing a plurality of inlet CO gas detectors,arranging the plurality of inlet CO gas detectors on the same gas inletand/or on inlets of different spray drying system components, and a stepof providing a plurality of inlet CO gas content measurements.

An optional step of providing the outlet CO gas detector and/or theinlet CO gas detector with process temperature compensation and/orprocess pressure compensation prior to the step 1200 of providing theoutlet CO gas content measurement and/or prior to the step 1000 ofproviding the inlet CO gas content measurement.

In the following the mathematical model lying behind the comparisonperformed by the analyzing unit 5 and forming part of the methodperformed by the powder drying system according to the invention, andmore particularly forming part of the step of comparing of the methodaccording to the invention will be described.

A rise in the CO gas concentration in the inlet air, C_(inlet), willover time result in an elevated CO gas concentration in the outlet air,C_(outlet). By mathematically modelling the spray drying system as aperfectly mixed tank plus a plug flow, t_(p), i.e. a time delay, itbecomes possible to calculate the CO gas concentration in the outlet airbased on the CO gas concentration in the inlet air. The calculated COgas concentration in the outlet air, C_(outlet), is indicative of thenaturally occurring fluctuations of the CO gas concentration in reactionto which the alarm system should not set off. Rather, the alarm systemshould only be set off in reaction to a rise in CO gas concentrationoriginating from smoldering of powders in the spray drying system. Therise in CO gas concentration, or differential measurement, ΔC_(outlet),originating from smoldering may be expressed as:

ΔC _(outlet) =C _(outlet,measured) −C _(outlet)   (1),

where C_(outlet), measured is the CO gas concentration in the outletaccording to an outlet CO gas content measurement measured by the outletCO gas detector.

To calculate C_(outlet) the following model may be set up. First of all,the amount of CO gas accumulated per time unit in the spray dryingsystem may be expressed as:

C _(inlet)*FLOW_(inlet) −C _(outlet)*FLOW_(outlet)   (2),

and as

$\begin{matrix}{\frac{{dC}_{{spray}\mspace{11mu} {dryer}}}{dt} \star {V_{{spray}\mspace{11mu} {dryer}}.}} & (3)\end{matrix}$

In the above equations FLOW^(let) is the air flow in m³/s in the mainair inlet 201, FLOW_(outlet) is the total gas flow in m³/s out of thespray drying system and V_(spraydryer) is the volume of the spray dryingsystem in m³.

The air flow in m³/s in the main air inlet 201, FLOW_(inlet), is assumedto be constant over time, is equal to the flow in the main air inlet inthe case that the static fluid bed and the VIBRO-FLUIDIZER™ have airinlets separate from the main air inlet, and is equal to the sum of theflow in each of the main air inlet, the air inlet of the static fluidbed and the air inlet of the VIBRO-FLUIDIZER™ if a common air inlet isused.

The total gas flow out of the spray drying system, FLOW_(outlet), iscalculated as the sum of the gas flow in each of the main gas outlet,the gas outlet of the static fluid bed and the gas outlet of theVIBRO-FLUIDIZER™ in m³/s.

The volume of the spray dryer in m³, V_(spraydryer), may be determinedexperimentally by introducing CO gas into the spray dryer. A theoreticalvalue may be obtained as t_(p)*FLOW_(inlet), where t_(p) is an assumedtime delay of the gas flow through the spray drying system, i.e. thetime it is assumed the gas flow takes to flow from the inlet CO gasdetector through the spray dryer to the outlet CO gas detector.

Due to the assumed perfect mixing in the spray dryer, the CO gasconcentration in the outlet air is equal to the CO gas concentration inthe spray drying system (dC_(spray dryer)/dt=dC_(outlet)/dt). Due to theassumed time delay, t_(p), of the gas flow through the spray dryingsystem, the inlet CO gas concentration measured by the inlet CO gasdetector at the time t_(p) before a given point of time, x, is used forcalculating the outlet CO gas concentration of the spray drying systemat the time x:

$\begin{matrix}{{\frac{{dC}_{outlet}(x)}{dt} = \frac{{{C_{inlet}\left( {x - t_{p}} \right)} \star {FLOW}_{inlet}} - {{C_{outlet}(x)} \star {FLOW}_{outlet}}}{V_{{spray}\mspace{11mu} {dryer}}}},} & (4)\end{matrix}$

where C_(inlet)(x-t_(p)) is the concentration of CO gas in ppm measuredby the inlet CO gas detector in the inlet at a number of seconds t_(p)before the time x.

Using sampling steps of a small size, t_(s), one obtains based onequation (4) above:

$\begin{matrix}{{\frac{{C_{outlet}(x)} - {C_{outlet}\left( {x - t_{s}} \right)}}{t_{s}} = \frac{{{C_{inlet}\left( {x - t_{p}} \right)} \star {FLOW}_{inlet}} - {{C_{outlet}(x)} \star {FLOW}_{outlet}}}{V_{{spray}\mspace{11mu} {dryer}}}},} & (5)\end{matrix}$

Thus, by rearranging equation (5), the following relation for thecalculated CO gas concentration in ppm in the outlet air, C_(outlet), ata time x is arrived at:

$\begin{matrix}{{{C_{outlet}(x)} = \frac{{\left( \frac{{c_{inlet}\left( {x - t_{p}} \right)} \star {FLOW}_{inlet}}{V_{spraydyer}} \right) \star t_{s}} + {C_{outlet}\left( {x - t_{s}} \right)}}{1 + \left( \frac{{FLOW}_{outlet} \star t_{s}}{V_{spraydryer}} \right)}},} & (6)\end{matrix}$

where C_(outlet)(x-t_(s)) is the concentration of CO gas in ppm at thetime x minus t_(s)−the sampling time−i.e. the latest previouslycalculated value of C_(outlet).

Equation (6) may thus be used for comparing the inlet CO gas contentmeasurement and the outlet CO gas content measurement. The result maythen be inserted into equation (1) together with the outlet CO gascontent measurement, C_(outlet, measured)(x), obtained by the outlet COgas detector at the time x to calculate the differential measurement,ΔC_(outlet)(x), indicative of the rise in CO gas concentrationoriginating from smoldering at the time x, thus achieving:

ΔC _(outlet)(x)=C _(outlet,measured)(x)−C _(outlet)(x)   (7).

Based on the thus calculated differential measurement, ΔC_(outlet),indicative of the rise in CO gas concentration originating fromsmoldering it may, for each calculation made, be determined whether analarm should be set off and/or other suitable action be taken.

It should be noted that the above description of preferred embodimentsserves only as an example, and that a person skilled in the art willknow that numerous variations are possible without deviating from thescope of the claims.

For instance, and optionally, as methane (CH₄) has adsorptionwavelengths being similar to those of CO, the method and/or deviceaccording to the invention may also be employed to detect CH₄ gas in agas inlet or gas outlet of a spray drying component. The results of sucha CH₄ gas detection may also be used to take into account the effectscaused by the CH₄ gas interfering with the CO gas measurements, at leastin case reasonably large quantities of CH₄ gas are present.

1. A powder drying system comprising a CO gas detection system adaptedfor detection of CO gas from smoldering powders in a powder dryingsystem component, such as a spray dryer chamber, a fluid bed, or a bagfilter, wherein the CO gas detection system comprises: at least oneinlet CO gas detector arranged on at least one gas inlet of the powderdrying system component such as to provide at least one inlet CO gascontent measurement; at least one outlet CO gas detector arranged on atleast one gas outlet of the powder drying system component such as toprovide at least one outlet CO gas content measurement; and an analyzingunit adapted for receiving the at least one inlet CO gas contentmeasurement from the at least one inlet CO gas detector, receiving theat least one outlet CO gas content measurement from the at least oneoutlet CO gas detector and comparing the sum of the at least one inletCO gas content measurement and the sum of the at least one outlet CO gascontent measurement while compensating for dilution, mixing, and timedelay of the outlet CO gas content measurement such as to provide adifferential measurement, ΔC_(outlet), indicative of the CO gas contentfrom smoldering powders in the powder drying system component, whereinat least the at least one outlet CO gas detector comprises an IR lasertransmitter and is adapted to detect over a measurement volume and isarranged on the at least one gas outlet in such a way that saidmeasurement volume extends directly inside a gas flow in said at leastone gas outlet.
 2. A powder drying system according to claim 1, whereinthe at least one inlet CO gas detector also comprises an IR lasertransmitter and is adapted to detect over a measurement volume and isprovided on the at least one gas inlet in such a way that itsmeasurement volume extends directly inside a gas flow in said gas inlet.3. A powder drying system according to claim 1, wherein at least the atleast one outlet CO gas detector and/or at least the at least one inletCO gas detector comprises an IR laser transmitter, an IR receiver, theIR laser transmitter and the IR receiver being arranged on positionsbeing one of mutually offset in a radial and/or longitudinal direction,mutually opposite and diametrically opposite such that its measurementvolume extends between said positions.
 4. A powder drying systemaccording to claim 1, wherein the measurement volume of the outlet COgas detector and/or the measurement volume of the inlet CO gas detectorextends over a length of at least 1 meter.
 5. A powder drying systemaccording to claim 1, wherein the at least one outlet CO gas detectorand/or the at least one inlet CO gas detector further comprises at leastone purging device arranged and adapted to purge the outlet CO gasdetector prior to providing the outlet CO gas content measurement and/orthe inlet CO gas detector prior to providing the inlet CO gas contentmeasurement.
 6. A powder drying system according to claim 1, wherein themeasurement volume of the outlet CO gas detector and/or the measurementvolume of the inlet CO gas detector may extend along any one of: aradial direction of the gas inlet or gas outlet, a longitudinaldirection of the gas inlet or gas outlet perpendicular to the radialdirection, or a direction being inclined with respect to thelongitudinal direction and/or the radial direction of the gas inlet orgas outlet.
 7. A powder drying system according to claim 1, furthercomprising a plurality of outlet CO gas detectors arranged on the samegas outlet and/or on outlets of different components of said powderdrying system such as to provide a plurality of outlet CO gas contentmeasurements, and/or a plurality of inlet CO gas detectors arranged onthe same gas inlet and/or on inlets of different components of saidpowder drying system such as to provide a plurality of inlet CO gascontent measurements.
 8. A powder drying system according to claim 1,wherein the analyzing unit is adapted to compare the inlet CO gascontent measurement and the outlet CO gas content measurement to obtaina differential measurement, ΔC_(outlet)(x), indicative of the CO gascontent from smoldering powders in a powder drying system at a giventime x by means of the relation:ΔC_(outlet)(x)=C_(outlet,measured)(x)−C_(outlet)(x), where:${{C_{outlet}(x)} = \frac{{\left( \frac{{c_{inlet}\left( {x - t_{p}} \right)} \star {FLOW}_{inlet}}{V_{spraydyer}} \right) \star t_{s}} + {c_{outlet}\left( {x - t_{s}} \right)}}{1 + \left( \frac{{FLOW}_{outlet} \star t_{s}}{V_{spraydryer}} \right)}},$and where: C_(outlet,measured)(x) is the concentration of CO gas in ppmin the gas outlet according to an outlet CO gas content measurement asmeasured by means of the outlet CO gas detector at the time x,C_(outlet)(x) is the calculated concentration of CO gas in ppm in thegas outlet at the time x, C_(outlet)(x-t_(s)) is the calculatedconcentration of CO gas in ppm in the gas outlet at the time x minus thesampling time t_(s), i.e. the latest previously calculated value of theconcentration of CO gas in ppm in the gas outlet, C_(inlet)(x-t_(p)) isthe concentration of CO gas in ppm in the gas inlet according to aninlet CO gas content measurement as measured by means of the inlet COgas detector a number of seconds t_(p) before the time x, V_(spraydryer)is the volume of the spray dryer in m³, FLOW_(inlet) is the air flow inm³/s in the main air inlet, FLOW_(outlet) is the total gas flow out ofthe spray dryer, calculated as the sum of the gas flow in each of themain gas outlet, the gas outlet of the static fluid bed and the gasoutlet of the vibro-fluidizer in m³/s, and t_(s) is the sampling time.9. A method for detecting CO gas from smoldering powder in a powderdrying system component, such as for instance a spray dryer chamber, afluid bed or a bag filter, of a powder drying system comprising a carbonmonoxide (CO) gas detection system, the method comprising: providing aninlet CO gas content measurement by means of an inlet CO gas detectorarranged on a gas inlet of the powder drying system component, providingan outlet CO gas content measurement by means of at least one outlet COgas detector arranged on a gas outlet of the powder drying systemcomponent, receiving the inlet CO gas content measurement from the inletCO gas detector, receiving the outlet CO gas content measurement fromthe at least one outlet CO gas detector, comparing the inlet CO gascontent measurement and the outlet CO gas content measurement tocompensate for dilution, mixing, and time delay of the outlet CO gascontent measurement such as to provide a differential measurement,ΔC_(outlet), indicative of the CO gas content from smoldering powders inthe powder drying system component, wherein the at least one outlet COgas detector comprises an IR laser transmitter and is adapted to detectover a measurement volume and is, prior to providing the outlet CO gascontent measurement, arranged on the gas outlet in such a way that itsmeasurement volume extends directly inside a gas flow in the gas outlet.10. A powder drying system according to claim 1, wherein at least the atleast one inlet CO gas detector and/or outlet CO gas detector comprisesan IR laser transmitter and an IR receiver and at least one reflector,the IR laser transmitter and the IR receiver being arranged in the sameposition and the at least one reflector being arranged in a positionbeing one of offset in a radial and/or longitudinal direction, mutuallyopposite and diametrically opposite with respect to the position of theIR laser transmitter and/or the IR receiver such that the measurementvolume of the at least one inlet and/or outlet CO gas detector systemextends between said positions.